Introduction

Modified atmosphere packaging (MAP) may be defined as 'the enclosure of food products in gas-barrier materials, in which the gaseous environment has been changed' (Young et al, 1988). Because of its substantial shelf-life extending effect, MAP has been one of the most significant and innovative growth areas in retail food packaging over the past two decades. The potential advantages and disadvantages of MAP have been presented by both Farber (1991) and Parry (1993), and summarised by Davies (1995) in Table 16.1.

There is considerable information available regarding suitable gas mixtures for different food products. However, there is still a lack of scientific detail regarding many aspects relating to MAP. These include:

Mechanism of action of carbon dioxide (CO2) on microorganisms.

Safety of MAP packaged food products.

Interactive effects of MAP and other preservation methods.

The influence of CO2 on the microbial ecology of a food product.

The effect of MAP on the nutrional quality of packaged food products.

16.2 Principles of MAP 16.2.1 General principles

Modified atmosphere packaging can be defined as packaging a product in an atmosphere that is different from air. This atmosphere can be altered in four different ways:

Table 16.1 The potential positive and negative effects MAP has on the food industry

Benefits

Disadvantages

1.

Product packaging

A centralised packaging system incorporating portion control Clear, all-round visibility of the product, improving its presentation characteristics

Increased package volume, adds to the transport costs and affects area required for retail display Benefits are lost when the package leaks or is opened

2.

Product quality

Overall product quality is high Sliced products are much easier to separate Shelf life increases by 50-400%

Product safety has not yet been fully established

3.

Special features

Use of chemical preservatives can be reduced or discontinued

Temperature control is essential Different products require their own specific gas formulation Speciality equipment and associated training is required

4.

Economics

Improved shelf life decreases financial losses Distribution costs are reduced due to fewer deliveries being necessary over long distances

Increased costs

after Davies, 1995.

after Davies, 1995.

Vacuum packaging.

Passive MAP.

Introduction of a gas at the moment of packaging.

Active packaging. In passive MAP, the modified atmosphere is created by the packaged commodity that continues its respiration after packaging. Active packaging systems alter the atmosphere using packaging materials or inserts absorbing and/or generating gases. Typical examples are oxygen absorbers and CO2 emitting films or sachets.

The gases that are applied in MAP today are basically O2, CO2 and N2. The last has no specific preservative effect but functions mainly as a filler gas to avoid the collapse that takes place when CO2 dissolves in the food product. The functions of CO2 and O2 will be discussed in more detail.

16.2.2 Carbon dioxide as anti-microbial gas

CO2, because of its antimicrobial activity, is the most important component in applied gas mixtures. When CO2 is introduced into the package, it is partly dissolved in the water phase and the fat phase of the food. This results, after equilibrium, in a certain concentration of dissolved CO2 ([CO2]diss) in the water phase of the product. Devlieghere et al (1998) have demonstrated that the growth inhibition of microorganisms in modified atmospheres is determined by the concentration of dissolved CO2 in the water phase.

The effect of the gaseous environment on microorganisms in foods is not as well understood by microbiologists and food technologists as are other external factors, such as pH and aw. Despite numerous reports of the effects of CO2 on microbial growth and metabolism, the 'mechanism' of CO2 inhibition still remains unclear (Dixon and Kell, 1989; Day, 2000). The question of whether any specific metabolic pathway or cellular activity is critically sensitive to CO2 inhibition has been examined by several workers. The different proposed mechanisms of action are:

Lowering the pH of the food.

Cellular penetration followed by a decrease in the cytoplasmic pH of the cell.

Specific actions on cytoplasmic enzymes.

Specific actions on biological membranes.

When gaseous CO2 is applied to a biological tissue, it first dissolves in the liquid phase, where hydration and dissociation lead to a rapid pH decrease in the tissue. This drop in pH, which depends on the buffering capacity of the medium (Dixon and Kell, 1989), is not large in food products. In fact, the pH drop in cooked meat products only amounted to 0.3 pH units when 80% of CO2 was applied in the gas phase with a gas/product volume ratio of 4:1 (Devlieghere et al, 2000b). Several studies have proved that the observed inhibitory effects of CO2 could not solely be explained by the acidification of the substrate (Becker, 1933; Coyne, 1933).

Many researchers have documented the rapidity with which CO2 in solution penetrates into the cell. Krogh (1919) discovered that this rate is 30 times faster than for oxygen (O2), under most circumstances. Wolfe (1980) suggested the inhibitory effects of CO2 are the result of internal acidification of the cytoplasm. Eklund (1984) supported this idea by pointing out that the growth inhibition of four bacteria obtained with CO2 had the same general form as that obtained with weak organic acids (chemical preservatives), such as sorbic and benzoic acid. Tan and Gill (1982) also found that the intracellular pH of Pseudomonas fluorescens fell by approximately 0.03 units for each 1 mM rise in extracellular CO2 concentration.

CO2 may also exert its influence upon a cell by affecting the rate at which particular enzymatic reactions proceed. One way this may be brought about is to cause an alteration in the production of a specific enzyme, or enzymes, via induction or repression of enzyme synthesis (Dixon, 1988; Dixon and Kell, 1989; Jones, 1989). It was also suggested (Jones and Greenfield, 1982; Dixon and Kell, 1989) that the primary sites where CO2 exerts its effects are the enzymatic car-boxylation and decarboxylation reactions, although inhibition of other enzymes has also been reported (Jones and Greenfield, 1982).

Another possible factor contributing to the growth-inhibitory effect of CO2 could be an alteration of the membrane properties (Daniels et al, 1985; Dixon and Kell, 1989). It was suggested that CO2 interacts with lipids in the cell mem brane, decreasing the ability of the cell wall to uptake various ions. Moreover, perturbations in membrane fluidity, caused by the disordering of the lipid bilayer, are postulated to alter the function of membrane proteins (Chin et al, 1976; Roth, 1980).

Studies examining the effect of a CO2 enriched atmosphere on the growth of microorganisms are often difficult to compare because of the lack of information regarding the packaging configurations applied. The gas/product volume ratio and the permeability of the applied film for O2 and CO2 will influence the amount of dissolved CO2 and thus the microbial inhibition of the atmosphere. For this reason, the concentration of dissolved CO2 in the aqueous phase of the food should always be measured and mentioned in publications concerning MAP (Devlieghere et al, 1998).

Only a few publications deal with the effect of MAP on specific spoilage microorganisms. Gill and Tan (1980) compared the effect of CO2 on the growth of some fresh meat spoilage bacteria at 30°C. Molin (1983) determined the resistance to CO2 of several food spoilage bacteria. Boskou and Debevere (1997;1998) investigated the effect of CO2 on the growth and trimethylamine production of Shewanella putrifaciens in marine fish, and Devlieghere and Debevere (2000) compared the sensitivity for dissolved CO2 of different spoilage bacteria at 7 °C. In general, Gram-negative microorganisms such as Pseudomonas, Shewanella and Aeromonas are very sensitive to CO2. Gram-positive bacteria show less sensitivity and lactic acid bacteria are the most resistant. Most yeasts and moulds are also sensitive to CO2. The effect of CO2 on psychrotrophic food pathogens is discussed in section 16.5.

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